remove overloaded to avoid c++17 dependency

include/ginkgo/core/base/utils_helper.hpp

@@ -514,29 +514,6 @@ class null_deleter<T[]> {
 };
 
 
-/**
- * Overloaded idiom for use with std::visit.
- *
- * Typical use case:
- * ```c++
- * std::variant<int, float> v;
- * std::visit(overloaded{
- *   [](int) { cout << "int";},
- *   [](float) { cout << "float";};
- *   }, v);
- * ```
- * Return values are also supported. Note that it is critical to use curly-brace
- * initializer for overloaded.
- */
-template <class... Ts>
-struct overloaded : Ts... {
-    using Ts::operator()...;
-};
-// explicit deduction guide (not needed as of C++20)
-template <class... Ts>
-overloaded(Ts...) -> overloaded<Ts...>;
-
-
 }  // namespace gko